1. Field of Invention
The invention relates generally to a method and system for controlling Raman gain flatness sensitivity to pump laser wavelength variations in an optical communications system utilizing Raman amplification.
2. Description of Related Art
Raman amplifiers are used in optical communications networks in applications such as ultra long haul transmissions. Energy from pump lasers is transferred to the signal carrying wavelengths through stimulated Raman scattering (SRS) to thereby amplify the signal.
In wavelength division multiplexed (WDM) systems, it is known to be desirable to provide uniform gain across multiple channels, a concept referred to as gain flatness. Unfortunately, conventional Raman amplifiers may generate non-uniform gain due to a number of factors. First, the pump lasers will have variance in the output wavelength due to manufacturing tolerances. Typically, pump laser manufacturers can deliver pump lasers with xc2x11.0 nm tolerance at room temperature (25xc2x0 C.). In addition to pump laser wavelength tolerances, the temperature dependence of Raman amplifier components can shift the pump laser wavelength significantly over xe2x88x925xc2x0 C. to 55xc2x0 C. ambient temperature. Existing Raman amplifiers may experience a temperature dependence of xcx9c0.02 nm/xc2x0 C., which corresponds to a xcx9c1.2 nm shift over xe2x88x925xc2x0 C. to 55xc2x0 C. ambient temperature. Thus, the worst case of pump laser wavelength deviation can be xc2x11.6 nm with the combination of wavelength tolerances from pump manufacturers and temperature dependent effects on laser wavelength.
FIG. 1 illustrates the deleterious effect of wavelength variance on gain flatness. As shown in FIG. 1, the Raman gain varies widely with variance in pump laser wavelength. These wavelength variances degrade the distributed Raman amplifier performance due to the increased gain ripple (i.e., reduced gain flatness) and eventually limit the transport distance of signals on the transmission path.